Sight glass



FIG. 1 is a left front perspective view of a sight glass showing my new design, the bottom is a mirror image of the top;

FIG. 2 is a front elevational view thereof, the undisclosed rear is unornamented and not part of the claimed design; and,

FIG. 3 is a right side elevational view thereof, the left side is a mirror image. 

The ornamental design for a sight glass, as shown and described. 